The present invention relates to a method of operating a multiple hearth furnace.
A multiple hearth furnace comprises an upright cylindrical furnace housing that is divided by a plurality of vertically spaced hearth floors in vertically aligned hearth chambers. A vertical shaft extends axially though the cylindrical furnace housing, passing centrally through each hearth floor. In each hearth chamber at least one rabble arms is secured to the vertical shaft and extends radially outside therefrom over the hearth floor. These rabble arms are provided with rabble teeth, which extend down into the material being processed on the respective hearth floor. As the vertical shaft rotates, the rabble arms move over the material on their respective hearth floor, wherein their rabble teeth plough through the material. The orientation of the rabble teeth of a rabble arm is such that they confer to the material a circumferential and a radial motion component, wherein the radial motion component is either centripetal (i.e. the material will be moved radially inwardly towards the vertical shaft) or centrifugal (i.e. the material will be moved radially outwardly towards the outer shell of the furnace). Drop holes are provided in each hearth floor, alternately in the inner zone of the hearth floor (i.e. centrally around the vertical shaft) or in the outer zone of the hearth floor (i.e. peripherally around the outer shell of the furnace). On hearth floors with a central drop hole, the rabble arms urge the material from the outer periphery of the hearth floor radially inwardly. On hearth floors with a peripheral drop hole, the rabble arms urge the material from the inner periphery of the hearth floor radially outwardly.
Operation of such a multiple hearth furnace takes place in the following manner. Solid material to be processed is supplied continuously via a material feed inlet into the uppermost hearth chamber, where it falls for example upon the outer periphery of the uppermost furnace floor. As the vertical shaft rotates, the rabble arms in the uppermost hearth chamber gradually urge the material in a kind of spiral movement over the hearth floor towards a central drop hole surrounding the vertical shaft. Through this central drop hole the material drops down onto the second hearth floor in the second hearth chamber, where the rabble arms of this chamber gradually work the material toward the outer periphery of the second hearth floor. Here the material drops through the peripheral drop holes of this second hearth floor onto the third hearth floor in the third hearth chamber. The material is then worked in the same way through successive hearth chambers, before it ultimately leaves the furnace via a material outlet in the hearth floor of the lowermost hearth chamber. Process gases move in an ascending counter-flow through the multiple hearth furnace. As the material travels downwards from hearth floor to hearth floor, it is thoroughly stirred and exposed to the hot process gases.
To optimise the process in the multiple hearth furnace, it is often of interest to feed additional material, e.g. a reducing agent as coal, on a lower hearth floor. This additional material is usually discharged by a conveyor through the outer shell of the furnace on a peripheral area of a hearth floor with a central drop hole (i.e. the rabble arms are consequently designed to urge the solid material radially inwardly, and the hearth floor immediately above has consequently peripheral drop holes). The rotating rabble arms urge the material falling through the peripheral drop holes of the next higher hearth floor and the additional material discharged by the conveyor through the outer shell of the furnace together to the central drop hole. Due to the ploughing action of the rabble teeth, both materials are thoroughly mixed before they fall through the central drop hole on the next lower hearth floor.
In many cases it would be of interestxe2x80x94at least from the point of view of process optimisationxe2x80x94to thermally precondition the additional material before adding it to the material already processed on upper hearth floors. Such a thermal preconditioning can for example comprise a preheating of the additional material to avoid an inhomogeneous temperature profile in the material bed, a preheating to dry the additional material or to evaporate other volatile components. However, in practice such a thermal preconditioning is generally not carried out, because it is considered to be too expensive in comparison to its benefits.
A problem addressed by the present invention is to provide a simple and inexpensive method for thermally preconditioning a additional solid material prior to adding it to a material already processed on upper hearth floors of a multiple hearth furnace.
In accordance with the present invention, a method of operating multiple hearth furnace with a plurality of vertically aligned hearth floors, comprises in particular following steps. A first material is fed onto the uppermost hearth floor and moved over this uppermost hearth floor before it falls through a drop hole onto the next lower hearth floor. This first material is processed in this way from hearth floor to hearth floor down to the lowermost hearth floor. A second material is fed onto one of the hearth floors to be mixed into the first material. In accordance with an important aspect of the present invention, the second material is moved separately from the first material in a separate annular zone of the hearth floor onto which it is fed before it is mixed into the first material. It will be appreciated that this method allows to provide an efficient thermal preconditioning of the second material prior to mixing it into the first material without requiring any supplementary equipment therefore.
In a generally preferred implementation of the method, the second material is fed onto an outer annular zone of a hearth floor, and the first material is dropped from a higher hearth floor onto an inner annular zone of this hearth floor. The first material is then moved in the inner annular zone of the hearth floor, and the second material is moved in the outer annular zone surrounding the first material in the inner annular zone. It will be appreciated that this way of proceeding allows to easily feed the second material through a lateral outer wall of the furnace onto the respective hearth floor.
For process reasons it may be of interest to keep the first and second material separate until they are dropped onto the next hearth floor. If this is the case, the second material is e.g. advantageously fed onto the outer periphery of the outer annular zone and moved inwardly towards the inner annular zone; whereas the first material is dropped onto the inner periphery of the inner annular zone and moved outwardly towards the outer annular zone. The first material and the second material can then be dropped through at least one common drop hole located in a fringe range between the inner and outer annular zones.
The first material and the second material may be dropped through the at least one common drop hole either onto an inner zone or onto an outer zone of a lower hearth floor. Here they are mixed by moving them from the inner zone to the outer zone, respectively from the outer zone to the inner zone, e.g. by means of rotating rabble arms as commonly used in multiple hearth furnaces.
The same rabble arms may be used for moving the first material in the inner zone and the second material in the outer zone. In the inner zone, the rabble teeth are then arranged so as to move the first material outwardly. In the outer zone, the rabble teeth are then arranged so as to move the second material inwardly.
In an alternative implementation of the method in accordance with the present invention, the second material is fed onto an inner annular zone of a hearth floor and moved herein, and the first material is dropped onto an outer annular zone of the hearth floor and moved herein around the second material in the inner annular zone of the hearth floor. This implementation is of particular interest if the second material can be easily fed, e.g. by means of a cooled conveyor radially introduced into the hearth chamber or through a hollow central shaft of the multiple hearth furnace, onto the inner periphery of the inner annular zone.
If it is of interest to keep, in the above alternative implementation, the first and second material separate until they are dropped onto the next hearth floor, then it is of advantage to proceed as follows. The first material is dropped onto the outer periphery of the outer annular zone and moved inwardly towards the inner annular zone. The second material is the fed onto the inner periphery of the inner annular zone and moved outwardly towards the outer annular zone. The first material and the second material are dropped through at least one common drop hole located in a fringe range between the inner and outer annular zones. If rabble arms with rabble teeth are used for moving the first material and the second material, then it is sufficient to arrange the rabble teeth in the inner zone so as to move the second material outwardly and the rabble teeth in the outer zone so as to move the first material inwardly.
It will be appreciated that the above described method of operating a multiple hearth furnace can be advantageously used within the context of a process for recovering metals from dusts and sludges, including inter alia important amounts of iron, zinc and lead. Such a process is advantageously carried out in a multiple hearth furnace comprising a first furnace stage and a second furnace stage. Separate furnace atmospheres prevail in each furnace stage, and each stage has a plurality of vertically aligned hearth floors. The first material, i.e. the material that is fed onto the uppermost hearth floor of the first furnace stage, is a material comprising the metal oxides. The second material, that is the additional material that is fed onto one of the hearth floors, is a coal with volatile constituents. The first material is first subjected to mainly endothermic preconditioning processes in the first furnace stage. The coal is fed onto the lowermost hearth floor of the first furnace stage and moved thereon separately from the first material in a separate annular zone of this hearth floor, wherein most of its volatile constituents are driven off and burned in the first furnace stage. The preconditioned first material and the preconditioned coal are then fed through at least one material lock onto the uppermost hearth floor of the second furnace stage and thoroughly mixed thereon, so that the metal oxides are subjected to a reduction by the preconditioned coal. It will be appreciated that this method of operating the double stage hearth furnace allows to substantially improve the thermal balance of the process by using the combustion energy of the volatile constituents of the coal for the endothermic processes in the first furnace stage. At the same time it helps to avoid a start of the exothermic reduction process in the first furnace stage, which would disturb the separation result by reducing and evaporating e.g. the zinc in the first furnace stage instead of the second furnace stage. Furthermore, the method warrants an excellent preconditioning of the coal for the reduction process in the second furnace stage.
If the preconditioned second material can be mixed into the first material already on the hearth floor onto which the second material is fed, then it may be of advantage to proceed in accordance with one of the following implementations of the method in accordance with the invention. According to a first implementation, the second material is fed onto the outer periphery of the outer annular zone and moved separately inwardly towards the inner annular zone, where it is transferred from the outer annular zone into the outer periphery of the inner annular zone. The first material is dropped onto the outer periphery of the inner annular zone and moved together with the second material inwardly through the inner annular zone, wherein both materials are thoroughly mixed. The mixed materials are finally dropped through at least one common drop hole at the inner periphery of the inner annular zone. According to a second implementation, the second material is fed onto the inner periphery of the inner annular zone and moved separately outwardly towards the outer annular zone, where it is transferred from the inner annular zone into the inner periphery of the outer annular zone. The first material is dropped onto the inner periphery of the outer annular zone and moved together with the second material outwardly through the outer annular zone, wherein both materials are thoroughly mixed. The mixed materials are finally dropped through at least one common drop hole at the outer periphery of the outer annular zone.